Thunderstorm Ground Enhancement is an atmospheric phenomenon which describes a significant increase of the ground-level radiation during thunderstorm activity. This effect is primarily attributed to the acceleration of charged particles by strong electric fields within thunderclouds, which can lead to enhanced gamma radiation detectable at the Earth’s surface.
In the 1920s, Wilson introduced the theory that the dipole structure of thunderclouds could accelerate electrons toward the ground. However, this theory did not gain immediate acceptance. It was eventually validated about 60 years later, confirming that the electrical configuration of thunderclouds indeed has the capability to accelerate particles downward or upward.
Over the past decade, the majority of Thunderstorm Ground Enhancements (TGEs) have been detected at the Alikhanyan National Science Laboratory Cosmic Ray Division on Mt. Aragats, Armenia. Equipment which is installed at the station includes particle detectors with different energy thresholds, electric field mills, a lightning detection network, and weather stations.
Aragats Research Station of Cosmic Ray Division, A. Alikhanyan National Science Lab on mt Aragats (3200 m a.s.l) (copyrights Andranik Keshishyan)
TGEs are more frequently registered in May as the thunderstorm activity is very high in Armenia during that period. TGEs can include high-energy electrons, gamma rays, and neutrons, with durations ranging from a few minutes to several hours depending on the energy level of the particles involved. The flux of the lower-energy particles (less than 3 MeV) can last more than two hours, and enhancements with high-energy particles (with energies up to 40 MeV) from 1 to 10 min (Chilingarian, 2018). So, thunderclouds can act as natural accelerators, producing particle flux enhancements registered on the ground during thunderstorms.
The electric field during which particle enhancements are detected on the surface, can have either a positive or negative polarity. These enhancements are attributed to the microphysical processes involving cloud and precipitation particles within these storms. However, the reasons behind the polarity assignment have remained unclear until recently.
Illustration of full tripole structure for deep (and colder) convection with “negative” Thunderstorm Ground Enhancements (TGE) (right side), and bottom heavy tripole for shallow (and warmer) convection with “positive” TGE (left side). Source Williams E et al 2022 (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JD035957)
In a recent study by Williams et al. (2022), high-energy particle, electric-field, and radar observations have been combined and revealed new insights for these high-energy phenomena. Within the study they used altitude-resolved S-band radar observations of graupel (graupel is a form of precipitation, created through a process called riming) to highlight distinct differences in the structure of storms associated with “positive” and “negative” TGEs on Mount Aragats in Armenia. Their findings indicate that shallow stages of convection are associated with “positive” TGEs, while deep stages of convection are linked to “negative” TGEs. These results align with the temperature-dependent electric tripole structure of thunderclouds.
The study of Thunderstorm Ground Enhancement is important for advancing our fundamental understanding of atmospheric physics. It can also have practical implications in areas such as aviation safety, radio communication, and environmental monitoring. Future research is expected to delve deeper into the mechanisms behind TGE, exploring how varying atmospheric conditions and storm structures influence ground-level radiation enhancements, to measure vertical profiles of electric fields in TGEs, also to answer a question if there are storms which are not generating TGEs? Currently, the ongoing research in thunderstorm phenomena and related atmospheric processes continues to shed light on the complex interactions within thunderclouds and their ground-level effects. As technology and methodologies advance, we anticipate more detailed insights that will further unravel the mysteries of Thunderstorm Ground Enhancement.
In conclusion, TGEs represent a significant interaction between thunderstorm activity and ground-level radiation, highlighting the complex dynamics within thunderclouds and their capability to influence environmental radiation levels. Further research in this area continues to unravel the mechanisms behind TGEs and their implications for understanding atmospheric physics and environmental monitoring.
References:
- Williams E, Mailyan B, Karapetyan G, Mkrtchyan H. Conditions for energetic electrons and gamma rays in thunderstorm ground enhancements, Journal of Geophysical Research: Atmospheres, 2023
- Williams, E., Mkrtchyan, H., Mailyan, B., Karapetyan, G., & Hovakimyan, S. Radar Diagnosis of the Thundercloud Electron Accelerator. Journal of Geophysical Research: Atmospheres, 2022
- Chilingarian A., Hovsepyan G., Karapetyan T., Karapetyan G., Kozliner L., Mkrtchyan H., et al. Structure of thunderstorm ground enhancements. Phys. Rev. D 101, 122004, 22 June, 2020
- Chilingarian A., Mkrtchyan H. et al. Catalog of 2017 Thunderstorm Ground Enhancement (TGE) events observed on Aragats. Scientific Reports, Vol. 9, Article number: 6253, 2019
- Chilingarian, A. (2018). Long lasting low energy thunderstorm ground enhancements and possible Rn-222 daughter isotopes contamination. Physical Review D.
